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MG MGB Technical - Sparkplugs

posted this in the Midget section a couple of days ago--thought I'd give it a run here as well - interested in anyone's take on it--
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Sparkplugs
Never ever have I had this happen before--
Yesterday I went to help an old friend get his new hotrod going--It's a 6litre Corvette engine with a multi throttle body conversion
New plugs,leads,coils, fuel pump etc
So here we go to start it---instant start, couldn't believe it, then after about 30secs it puttered out and stopped---These motors have a reputation of crank sensors failing,so off with a plug lead and cranked, very healthy spark so crank sensor ok, checked fuel supply,ok, checked for injector pulse,ok
pulled a plug and checked spark at plug--no
These are brand new expensive Platinum resistor plugs and they died---all of them.
It appears the resistors in them failed.
New identical set, checked the gaps and reset to spec 1.5mm (60thou)fitted them up and away it went---
So what killed the others, they had been set at .7mm instead of 1.5mm so the only thing I can think of is that the monster output coils combined with the small gap sizled the resistors---but unreal that they all died together--measuring with a meter there is absolutely no circuit from the end post to the centre electrode---it's got me stuffed how this happened to be honest but happen it did-----Good news is that it's going like a train now -did probably 100klm in it giving it heaps and it's perfect---
Anyone struck that before--it's new to me--
willy
William Revit

Have you measured the resistance of a good one for comparison?

With a reduced gap the voltage at the plug will be lower than it would otherwise be, and I can't see that blowing the resistors. Sounds more like a bad batch, if you go back to the supplier you may well find they have had other complaints.

As I don't like unresolved issues - cause-wise - like this I'd be reducing the gap on one of the plugs that is working now and see what happens. If that failes as well I'd be taking it up with the manufacturer as well as the supplier. Even though the larger gap hasn't failed so far it may well do so in the not too distant future.
paulh4

Well Paul, this has got interesting
The car has done about 500klm now and still ok--pulled a couple of plugs for a look, nice biscuity brown colour so all good there
Measured the resistance of both of them and both had open circuits so I suspect the resistance in there is an air gap or similar
Closed the gap on one down to 25thou and put them back in for a run-started it up , ran ok jumped in and didn't make it to the street before poof poof poof out the exhaust, yep you guessed it, another $20 sparkplug down the gurgler,-replaced with yet another plug gapped to 60thou and all well again-----
I've got a mate with a high pressure water cutter, so next time I'm out his way I'll get one of the dud plugs cut down the middle,can't wait to see what's going on in there. I'll let you know what's happening-

William Revit

According to NGK their resistor plugs "contain a single ceramic monolithic resistor of approximately 5000 ohms. Because of the type and construction of the resistor (i.e. no springs), the problems of vibration and sudden changes in temperature that can occur with some other brands do not apply."

Presumably the second sentence relates to the potential (pun not intended) for other plugs that make the internal connections with springs, to go open-circuit.

I take it (apologetic cough) you measured between the centre electrode and the connection for the plug lead?
paulh4

apologetic cough accepted
Yes and measured in both directions in case there was some weird diode effect in there but nothing
and- they are NGK--wasn't going to mention a brand but it is what it is I guess
I'll get back to you
William Revit

Well Paul, this has got interesting
Took the plug out to get sliced and the water cutter that he uses to cut 2" thick hardened steel wouldn't cut the porcelain so I had to cut the metal off then smash it
What's left of the electrode measures 11,000 ohms and there is a gap where I reckon the electrode probably had a larger top that the post looks like it screwed into or up against
I've laid it out here the length that it sat in the plug ,the top of the electrode has simply burnt away
Why it would do this with the small plug gap and not the large has got me stuffed to be honest


William Revit

Interesting as you say. At the same time as this there is a post on the MGOC forum about pretty much the same thing, with NGK BPR6RES plugs seeming to cause a misfire.

Got a bit confusing as at first he claimed they were non-resistor, but in his pictures the BPR was clearly visible. When shown that he agreed that he did have two resistor and two non-resistor supplied as a set (all replaced), but then said he couldn't test them for resistance (as asked originally) as they were in the 'dumpster'. So how he knew what had been in after they had been dumped, or why he couldn't test them if he had been able to see that two were resistor, I don't know.

Illogicality like that really upsets my CDO (CDO is like OCD but the letters have to be in alphabetical order ...).
paulh4

These are---NGK pztr5a-15
William Revit

Willy,
why not send the photo and info to NGK and ask their technical department. I've usually found most manufacturers at least reply and many are helpful with supplying an answer.
Nigel Atkins

Might just do that Nigel, I'll wait a bit just to make sure it keeps going ok with the propper gaps but at this stage it's starting and going perfectly
William Revit

I'm not good on this sort of electrical stuff, but could using resistor leads have any bearing on the problem, or maybe CDI systems?
Allan Reeling

I'm wondering if it might be a trade-off between voltage and current.

All coils generate a certain amount of energy, and with your 'monster output coils' this could well above what would be normal.

With large gaps the voltage will have to rise that much higher to jump that gaps, then a certain current will flow. With smaller gaps the voltage won't have to rise so high, so maybe the current increases as the energy will still be the same.

Whilst logically it doesn't seem very likely that the resistor construction would be so marginal as to make a difference, who knows ... NGK?

Another factor could be excessive external heat, given the appearance.

Resistor leads will tend to reduce the energy at the plug.
paulh4

These things have an individual high output coil for each plug which I believe to be around 50,000 volt output, they are mounted on the rocker cover with a short resistor plug lead each probably about 6" long, all as per a std. Corvette
I've just been on a Corvette forum and they're all canning the NGK plugs and fitting the std AC Delco's or Bosch
I've sent a message to a mate that works at Rousch to see what he's heard about it, if anyone knows ,he will,we'll see.
William Revit

This thread was discussed between 13/09/2018 and 19/09/2018

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